diff --git a/js/src/jsnum.cpp b/js/src/jsnum.cpp
index 74f8eb57dcc..5975fcd83bc 100644
--- a/js/src/jsnum.cpp
+++ b/js/src/jsnum.cpp
@@ -41,9 +41,6 @@
 /*
  * JS number type and wrapper class.
  */
-#if defined(XP_WIN) || defined(XP_OS2)
-#include <float.h>
-#endif
 #ifdef XP_OS2
 #define _PC_53  PC_53
 #define _MCW_EM MCW_EM
diff --git a/js/src/jsnum.h b/js/src/jsnum.h
index a4b8422b231..e521170e3ec 100644
--- a/js/src/jsnum.h
+++ b/js/src/jsnum.h
@@ -40,8 +40,10 @@
 #ifndef jsnum_h___
 #define jsnum_h___
 
-#include <float.h>
 #include <math.h>
+#if defined(XP_WIN) || defined(XP_OS2)
+#include <float.h>
+#endif
 
 /*
  * JS number (IEEE double) interface.
@@ -97,6 +99,17 @@ JSDOUBLE_IS_FINITE(jsdouble d)
 #endif
 }
 
+static inline int
+JSDOUBLE_IS_INFINITE(jsdouble d)
+{
+#ifdef WIN32
+    int c = _fpclass(d);
+    return c == _FPCLASS_NINF || c == _FPCLASS_PINF;
+#else
+    return isinf(d);
+#endif
+}
+
 static inline int
 JSDOUBLE_IS_NEGZERO(jsdouble d)
 {